1. Field of the Invention
The present invention relates to a technology to separate a document image into a plurality of areas. More particularly, the present invention relates a technology to detect characters, ruled lines and the like, and to recognize the characters, by separating areas including photographs and the like in a colored document image, for reproducing a document including information about layouts and colors.
2. Description of the Related Art
Accurate separation of areas on a document is necessary as preparation for recognizing characters on the document in an OCR (Optical Character Reader) process. Generally, binarization of an image of a color document is carried out in area separation for performing the OCR process or the like on the color document. However, because of such binarization, the area separation cannot be carried out with high accuracy. As described above, the binarization of an image is often used in an area of related-art character recognition. On the other hand, color information is not often used actively in the related-art character recognition. For instance, an image-processing device disclosed in Japanese Laid-open Patent Application No. 07-099581 initially binarizes a color image to create a binary image, and then, performs an area separation process or a character recognition process to the binary image, in a case in which the color image is inputted to the image-processing device.
Additionally, related-art technologies regarding area separation performed on a colored document image are described below.
Japanese Laid-open Patent Application No. 05-064016 discloses an image-compression device that separates natural pictures (photographs) and line drawings (characters, illustrations and graphs) for the purpose of compressing an input colored document image efficiently. However, Japanese Laid-open Patent Application No. 05-064016 does not disclose a technology to efficiently separate a text area and areas other than the text area. Therefore, a result of area separation according to the technology is hardly adapted to the OCR process on a color document.
Additionally, Japanese Laid-open Patent Application No. 06-339019 discloses a method of separating a text area and a photograph area on a document image by expressing the document image in a frequency element, and then, by using discrete cosine transformation. In the method, a color image having high resolution and high quality must be provided for accurately extracting coefficients that characterize a character and a photograph from discrete cosine transformation coefficients. Therefore, there is a chance of consuming an enormous disk-memory capacity as well as taking a long execution time, according to the method disclosed in Japanese Laid-open Patent Application No. 06-339019.
Additionally, Japanese Laid-open Patent Application No. 07-168939 discloses a colored-image area separation device separating a colored photograph area and an area including a figure having a few color variations or characters, by use of a hue histogram. However, Japanese Laid-open Patent Application No. 07-168939 focuses on a hue for separating the areas, and, thus, is ineffective to a monochrome or gray-scale document such as newspaper.
As described above, a number of methods regarding the OCR process are currently suggested. However, the most of the suggested methods are intended for a binary image. Thus, a condition of the binary image has large influence on recognition accuracy. On the other hand, a colored document has a tendency to have not only more color variations, but also a more complicated document layout. However, even if an area separation process according to a related-art method is performed on a binary image created from a color image, satisfactory accuracy in the area separation cannot be generally acquired.
A number of methods have been suggested for creating a binary image suitable for character recognition. A typical method of creating such a binary image is a discriminant analysis method. For instance, Japanese Laid-open Patent Application No. 10-143608 discloses a technology to create a binary image suitable for the OCR process, by taking the following steps. An image having shades is initially binarized by use of a threshold. Then, an average line width on the created binary image is calculated. If a value of the average line width is outside a regulated range, the created binary image is determined to be inappropriate for character recognition, and the binarization of the image is carried out again.
As described above, character recognition on a color document and reproduction of the color document including information about layouts and colors are hardly achieved by a related-art technology. In addition, accurate creation of a binary image suitable for the OCR process is hardly performed on a colored document, since the colored document has problems peculiar to colored documents.